As the number of mobile subscribers has been increasing constantly and most likely will continue to do so, a great number of users may be present in one geographically limited area (e.g. a cell of a communication network). This may cause issues with regard to capacity of the network. Thus, one of the targets of sophisticated radio communication networks, such as the Long Term Evolution (LTE) or the LTE-Advanced (LTE-A) of the 3rd Generation Partnership Project (3GPP), is to be able to accommodate several subscribers in a cell without severe problems in the connections between a serving eNB and the subscribers (user terminal, UT).
For example the above mentioned communication networks are planned to adopt multiple-input multiple-output (MIMO) transmission between the UT and the eNB in order to improve the throughput of the network. The UT may have, for example, two transmit/receive antennas, which are typically separated by at least one half of a wave length or have a different polarization direction. The multiple antennas can be used to transmit the same data to enhance reliability of the communication (transmit spatial diversity), or they can transmit different data to enhance the throughput of the transmission (data multiplexing). When a single user is transmitting/receiving data to/from the eNB by using multiple antenna elements, the communication may be called single user MIMO (SU-MIMO).
To further enhance the reliability of the transmission and also to decrease the power consumption of the device, the UT may apply beamforming with the multiple antennas. That is, instead of transmitting the data omni-directionally, the UT directs its transmission/reception antenna beam towards a certain direction. In order to do this, the data transmitted from the antenna elements of the UT need to be weighted with certain coefficients. The beam may be formed independently for each data stream to be transmitted. However, selecting the coefficients so as to maximize the communication throughput requires knowledge of the channel state information.
For this reason, channel sounding signals, such as the sounding reference signals (SRS), are transmitted in uplink and the eNB may obtain knowledge of the radio interface condition by receiving the signals. The eNB may then inform the UT about the appropriate antenna coefficients. However, as the SRSs for sounding the channel between the UT and the eNB have only a limited capacity, the use of SRS may not be enough in connection with a great number of simultaneous subscribers. Thus, it is important to provide a solution for improving the sounding of the channel.